SOLVENT PRESSURE EFFECTS IN FREE-RADICAL REACTIONS - A SELECTIVITY INVERSION IN FREE-RADICAL BROMINATIONS INDUCED BY SOLVENT

The results reported herein demonstrate a compelling link between selectivity in free radical brominations and the internal pressure of solvent. In a study encompassing 18 solvents and/or solvent mixtures, the rate constant ratio for S(H)2 ring opening of cyclopropylbenzene (k(C)) vs hydrogen abstraction from toluene (k(H)) by bromine atom was found to vary by nearly a factor of 20 as a function of solvent. The observed rate constant ratio correlates with the cohesive energy density of the solvent, which for the solvents utilized in this study is approximately equal to internal pressure (k(C)/k(H) is found to increase with increasing solvent pressure). A similar competition pitting cyclopropylbenzene against p-chlorotoluene shows an identical solvent effect. It is suggested that the variation in selectivity with solvent pressure arises because the volume of activation associated with k(C) is more negative than that for k(H) by ca. 20 cm3/mol.